Ionized physical vapor deposition is a process which has particular utility in filling and lining high aspect ratio structures on silicon wafers. In ionized physical vapor deposition (IPVD) for deposition of thin coatings on semiconductor wafers, materials to be deposited are sputtered or otherwise vaporized from a source and then a substantial fraction of the vaporized material is converted to positive ions before reaching the wafer to be coated. This ionization is accomplished by a high-density plasma which is generated in a process gas in a vacuum chamber. The plasma may be generated by magnetically coupling RF energy through an RF powered excitation coil into the vacuum of the processing chamber. The plasma so generated is concentrated in a region between the source and the wafer. Then electromagnetic forces are applied to the positive ions of coating material, such as by applying a negative bias on the wafer. Such a negative bias may either arise with the wafer electrically isolated by reason of the immersion of the wafer in a plasma or by the application of an RF voltage to the wafer. The bias causes ions of coating material to be accelerated toward the wafer so that an increased fraction of the coating material deposits onto the wafer at angles approximately normal to the wafer. This allows deposition of metal over wafer topography including in deep and narrow holes and trenches on the wafer surface, providing good coverage of the bottom and sidewalls of such topography.
Certain systems proposed by the assignee of the present application are disclosed in U.S. patent applications Ser. Nos. 08/844,751, now U.S. Pat. No. 5,878,423, 08/837,551 now U.S. Pat. No. 5,800,688, and 08/844,756 filed Apr. 21, 1997, allowed and hereby expressly incorporated by reference herein. Such systems include a vacuum chamber which is typically cylindrical in shape and provided with part of its curved outer wall formed of a dielectric material or window. A helical electrically conducting coil is disposed outside the dielectric window and around and concentric with the chamber, with the axial extent of the coil being a significant part of the axial extent of the dielectric wall. In operation, the coil is energized from a supply of RF power through a suitable matching system. The dielectric window allows the energy from the coil to be coupled into the chamber while isolating the coil from direct contact with the plasma. The window is protected from metal coating material deposition by an arrangement of shields, typically formed of metal, which are capable of passing RF magnetic fields into the interior region of the chamber, while preventing deposition of metal onto the dielectric window that would tend to form conducting paths for circulating currents generated by these magnetic fields. Such currents are undesirable because they lead to ohmic heating and to reduction of the magnetic coupling of plasma excitation energy from the coils to the plasma. The purpose of this excitation energy is to generate high-density plasma in the interior region of the chamber. A reduction of coupling causes plasma densities to be reduced and process results to deteriorate.
In such IPVD systems, material is, for example, sputtered from a target, which is charged negatively with respect to the plasma, usually by means of a DC power supply. The target is often of a planar magnetron design incorporating a magnetic circuit or other magnet structure which confines a plasma over the target for sputtering the target. The material arrives at a wafer supported on a wafer support or table to which RF bias is typically applied by means of an RF power supply and matching network.
A somewhat different geometry employs a plasma generated by a coil placed internal to the vacuum chamber. Such a system does not require dielectric chamber walls nor special shields to protect the dielectric walls. Such a system is described by Barnes et al. in U.S. Pat. No. 5,178,739, expressly incorporated by reference herein. Systems with coils outside of the chamber as well as the system disclosed in the Barnes et al. patent involve the use of inductive coils or other coupling elements, either inside or external to the vacuum, that are physically positioned and occupy space between the planes of the sputtering target and the wafer.
Whether a coupling element such as a coil is provided inside or outside of a vacuum chamber, dimensions of the system have been constrained by the need for adequate source to substrate separation to allow for the installation of the RF energy coupling elements between the source and the substrate. Adequate diameter must also be available around the wafer for installation of coils or other coupling elements. As a direct result of the increased source to substrate spacing due to the need to allow space for the coupling element, it is difficult to achieve adequate uniformity of deposition with such systems. If the height of the chamber is reduced to improve uniformity there is a loss of plasma density in the central region of the chamber and the percentage of ionization of the coating material is reduced. Also, in practice, the entire system must fit within a constrained radius. As a result, there are frequently problems due to heating arising from the proximity of the RF coils to metal surfaces, which may necessitate extra cooling, which increases engineering and production costs and wastes power.
An IPVD apparatus with the coil in the chamber has the additional disadvantage that the coils are eroded by the plasma and must therefore consist of target grade material of the same type as that being sputtered from the target.
Moreover, considerable cooling of coils placed in the vacuum chamber is needed. If liquid is used for this cooling of the coils, there is danger that the coils will be penetrated by uneven erosion or by arcing, causing a resulting leak of liquid into the system, which is highly undesirable and will likely result in a long period of cleaning and requalification of the system. Furthermore, an excitation coil in the chamber also couples capacitively to the plasma, leading to inefficient use of the excitation power and to the broadening of the ion energy spectrum, which may have undesirable effects on the process.
The miniaturization of semiconductor devices has resulted in a need to form low resistance connections to contacts at the bottoms of high aspect ratio holes of a fraction of a micron in diameter. This has increased the demand for the use of highly electrically conductive metals such as copper over barrier layers of materials such as tantalum and tantalum nitride. The techniques for depositing such materials in the prior art have not been totally satisfactory.
The deposition of materials by PVD methods has, in the prior art, involved critical designs of sputtering sources to produce plasma concentrations of uniform geometries within sputtering chambers and to directly affect the distribution uniformities of the deposited films. The prior art approaches have resulted in compromises of other performance parameters to those ends.
As a result of the above considerations and problems, there remains a need for more efficiently coupling energy into the dense coating material ionizing plasma in IPVD processing systems, and to do so without interfering with the optimum dimensions of the chamber and preferably without placing a coil or other coupling element into the vacuum chamber.